Leads implanted in the body for electrical cardioversion or pacing of the heart are generally known in the art. In particular, leads implanted in or about the heart have been used to reverse (i.e., defibrillate or cardiovert) certain life threatening arrhythmias, or to stimulate contraction (pacing) of the heart. Electrical energy is applied to the heart via the leads to return the heart to normal rhythm. Leads have also been used to sense in the atrium or ventricle of the heart and to deliver pacing pulses to the atrium or ventricle. Tachy leads can sense, pace, and deliver defib shocks. Brady leads pace and sense the heart. Technically, the pacemaker or the automatic implantable cardioverter defibrillator (AICD) receives signals from the lead and interprets them. In response to these signals the pacemaker can pace or not pace. The AICD can pace, not pace or shock, and not shock. In response to the sensed bradycardia or tachycardia condition, a pulse generator produces pacing or defibrillation pulses to correct the condition. The same lead used to sense the condition is sometimes also used in the process of delivering a corrective pulse or signal from the pulse generator of the pacemaker.
Sick sinus syndrome and symptomatic AV block constitute the major reasons for insertion of cardiac pacemakers today. Cardiac pacing may be performed by the transvenous method or by leads implanted directly onto the ventricular epicardium. Most commonly, permanent transvenous pacing is performed using a lead positioned within one or more chambers of the heart. A lead, sometimes referred to as a catheter, may be positioned in the right ventricle or in the right atrium through a subclavian vein, and the lead terminal pins are attached to a pacemaker which is implanted subcutaneously. The lead may also be positioned in both chambers, depending on the lead, as when a lead passes through the atrium to the ventricle. Sense electrodes may be positioned within the atrium or the ventricle of the heart.
Pacemaker leads represent the electrical link between the pulse generator and the heart tissue which is to be excited. These pacemaker leads include single or multiconductor coils of insulated wire having an insulating sheath. The coils provide a cylindrical envelope, many times referred to as a lumen, which provides a space into which a stiffening stylet can be inserted. The conductive coil is connected to an electrode in an electrode assembly at a distal end of a pacing lead. Typically, a terminal member is mounted within a flexure sleeve at the proximal end of the pacing lead and connected to the proximal end of the conductive coil.
After the electrode assembly is positioned at a desired location within the heart, it is desirable to provide some method for securing the electrode assembly at that location. Mechanical fixation devices are used to firmly anchor the electrodes in the heart. One type of mechanical fixation device used is a corkscrew, or a helix. During placement of the lead, the tip of the lead travels intravenously through veins and the heart. While traveling through the veins, the helix at the tip of the lead may snag or attach to the side wall of the vein. Since this is highly undesirable as it may cause damage or other complications to a patient, retractable helixes have been provided for leads.
The helix is extended and screwed into the heart muscle by applying a torque to the other end of the conductor without use of any further auxiliary device or with a special fixation stylet. A lead must be capable of being firmly secured into the wall of the cardiac tissue to prevent dislodgement therefrom, while avoiding perforation of the electrode completely through the cardiac tissue.
There is a need for a body-implantable lead that has a helix for fixation to the wall of the atrium or ventricle of the heart. In addition, there is a need for a lead having an electrode for positioning within the atrium or ventricle that allows for tissue ingrowth. Tissue ingrowth further enhances the electrical performance of the lead. The lead and electrode are further stabilized within the heart as a result of tissue ingrowth.